Deep in the South American jungle, a Japanese hunter named Ryosuke (Ryo for short), and a local boy named Chico, stalk a monster that has been terrorizing the local countryside. Ryo, however, is quite familiar with this beast, and the story flashes back to his childhood.
Teenage Ryosuke Ishigami, the delinquent son of a crime reporter and a geneticist, is out with a motorcycle gang when they encounter a mysterious woman. Some of the rougher members of the gang accost her, and she turns out to be anything but normal, landing the gang with serious injuries. The gang leader returns to the woman’s hideout for revenge, but the gang members are torn apart, except for Ryosuke.
The woman, named Bagi, turns out to be a “cat-woman” – a cross between a human and a mountain lion. She recognizes Ryosuke as the boy who had rescued her and raised her as a kitten. As Bagi grew and people became suspicious of the precocious “cat”, who was able to walk on her hind legs and even learned to write her own name and speak, she escaped and grew to adulthood on her own.
Upon their reunion, Ryosuke and Bagi join forces to find out the truth of her origins. Ryosuke’s own mother is found responsible for Bagi’s creation — Bagi is a product of recombinant DNA research between human and mountain lion cells. They then follow Ryosuke’s mother to South America to confront her about the reason for Bagi’s existence, but find a far greater peril. The officials in charge of the laboratories there are creating a strain of rice that has the potential to destroy humanity. Ryosuke’s mother sacrifices her life to have Bagi destroy the “Rice Ball” and Ryosuke mistakenly pins the blame on Bagi, vowing revenge.
Meanwhile, Bagi is quickly losing her human traits and becomes extremely feral, attacking any humans that come near. Ryo catches up to her and stabs her when she attacks, but then finds a hand-written note held in a locket around her neck. He reads his mother’s last words, expressing remorse for being both a bad scientist and a bad mother, and Ryo realizes his mistake and is filled with regret. He returns to the site the next morning to find Bagi’s body gone, a set of footprints leading off to the distant mountains. He prays for Bagi to live on in solitude, far away from mankind.